The present invention relates to a data decoding device used in a digital mobile communication receiver.
FIG. 1 shows a configuration of a prior art data decoding device. In FIG. 1, numeral 1 denotes a base station, numeral 2 denotes a transmission antenna thereof, numeral 3 denotes a receiving antenna and numeral 4 denotes a demodulator. Numeral 5 denotes a demodulated signal which is an output of the demodulator 4 and which is applied to an equalizer 6. An equalized signal 7 which is an output of the equalizer 6 is applied to a decoder 8 and an error calculator 9. Numeral 10 denotes a square error which is applied to a likelihood calculator 11. A likelihood 12 which is an output of the likelihood calculator 11 is applied to the decoder 8 which produces decoded data 13 and an estimated error rate 14.
FIG. 2 shows a configuration of the error calculator 9. In FIG. 2, an equalization signal 15 is supplied to a discriminator 16. An error 18 between a discrimination signal 17 which is an output of the discriminator 16 and the equalization signal 15 is calculated and a square error 19 which is a square of the error 18 is applied to an adder 20 which produces 1 a square error sum 21.
An operation of the prior art is now explained. In FIG. 1, the base station 1 transmits a signal to a mobile station from the transmission antenna 2. The mobile station receives it by the receiving antenna 3 and demodulates it by the demodulator 4 to produce the demodulated signal 5 to be used by the equalizer 6, which eliminates a distortion of a waveform of the demodulated signal 5 and improves an error rate to produce the equalization signal 7, which is supplied to the error calculator 9 and the demodulator 8. As shown in FIG. 2, the error calculator 9 discriminates a polarity of the equalization signal 15 by the discriminator 16, and if it is positive, it converts the equalization signal 15 to 1, and if it is negative, it converts the equalization signal to -1. Then, a difference between the equalization signal 15 and the identification signal 17 is calculated to produce the error 18. The error 18 is then squared to produce the square error 19 which is applied to the adder 20 to produce the square error sum 21.
When the equalization signal 15 does not include an error, the square error sum 21 is equal to a power of a noise component of the equalization signal 15 and an error rate of the equalization signal 15 may be derived therefrom. However; when an error is included, the square error sum 21 is not equal to the power and an error rate cannot be derived therefrom FIG. 3 shows the equalization signal 15 and the square error 19, and a probability of occurence thereof when the transmission data is -1 and the discrimination signal 17 is -1 (no error) even if the equalization signal 15 is positive. By calculating in the way, the square error sum 21 is equal to the power of the noise component of the equalization signal 15. In actual use, however, since the discrimination signal 17 is 1 when the equalization signal 15 is positive, the square error 19 appears as shown in FIG. 4 in which it is smaller than a real noise component. The square error 10 which is the output of the error calculator 9 is applied to the likelihood calculator 11 which calculates the likelihood 12. The decoder 8 makes soft discrimination decoding based on the likelihood 12 and the equalization signal 7 to produce the decoded data 13 having a low error rate. The decoder 8 further encodes the decoded data 13 to simulate it to a pattern transmitted by the base station, and compares it with the equalization signal 7 to produce an estimated error rate 14. (The deduced error 14 is required in reporting a quality of a line to the base station). As described above, the prior art data decoding device has the problems that (1) the reencoding by the decoder 8 is required because the error rate cannot be derived from the square error sum 10 since the error sum is smaller than the real noise component when the error is included; (2) the derived error rate includes an error because the encoded signal does not represent to the pattern transmitted from the base station when the error is included; and (3) the error rate of the decoding data is higher than that obtained when the error rate is used because it is considered that more appropriate redundancy may be obtained when the error rate is used in the likelihood calculation than when the square error sum 10 is used since the likelihood 12 should be added in accordance with the error rate.